Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in ink jet techniques.
Most of the ink jet printers known in the art may be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous ink jet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. While the vehicle in many continuous-type inks comprises water, U.S. Pat. No. 4,142,905, in the name of Cooke, disclosed a water-free ink composition consisting essentially of a low molecular weight glycol, an inorganic salt electrolyte, and a dye which is soluble in the glycol.
With so-called "impulse" or "drop-on-demand" ink jet printers, image formation is controlled by selectively energizing and de-energizing a piezoelectric transducer rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not bear an electric charge. Accordingly, impulse ink compositions are free of corrosive substances such as water and electrolytes which continuous stream inks often comprise.
However, impulse ink jet printers present a number of problems which are not encountered in continuous ink jet systems. For example, unlike continuous ink jet printers, impulse printers typically are maintained in a stand-by or temporarily inoperative mode between printing cycles. Thus, the ink is allowed to stand and possibly solidify in the discharge orifices of the print head. Impulse printers normally begin a printing cycle with such material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to stand in the discharge orifices during stand-by periods. Such material is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink. Even where ink is allowed to stand and solidify, it is more easily purged due to the considerably higher pressures at which continuous ink jet printers operate. Accordingly, impulse-type inks must be specially formulated to minimize start-up problems.
Numerous ink compositions for impulse ink jet printers are known in the art. However, many of these inks are not suitable for writing on porous or fibrous substrates. As will be appreciated by those of skill in the art, an ink applied to a substrate such as paper will tend to migrate or wick along the fibers of the paper until the wicking forces are countered by the ink viscosity. The extent to which the ink wicks will be dependent upon both its viscosity and the porosity of the substrate. Where highly porous substrates such as kraft paper or corrugated cardboard are employed, many inks tend to wick excessively, leading to blurry, ill-defined printed images. One approach to clearer, more well-defined print images has involved the employment of a rapidly evaporating ink vehicle. However, rapid evaporation of the vehicle of an impulse-type ink often leads to clogging of discharge orifices during stand-by periods. Moreover, rapid evaporation is undesired due to the adverse environmental and health effects of many commonly-employed ink vehicles.
In certain applications, it is necessary that the image created by an ink jet printer possess a relatively intense color. For example, many optical character reading devices cannot read images unless they possess a minimum color intensity. Those skilled in the art will recognize that bar code images typically must possess a print contrast signal (pcs) of about 90 or greater to be machine readable. However, many of the known techniques for increasing the color intensity of an ink--such as increasing the concentration of the colorant--often adversely affect important ink properties such as viscosity, surface tension, and stability.
Therefore, there exists a need for an impulse-type ink jet ink composition capable of producing clear, well-defined, color-intense images on even porous substrates.